The present invention generally relates to asphalt distributors and more particularly relates to control valves in the circulating systems of asphalt distributors.
Asphalt distributors apply hot liquid asphalt to road and other surfaces in a variety of paving applications. Upon cooling, asphalt material becomes more viscous and eventually xe2x80x9cfreezesxe2x80x9d to provide a binder material for pavement. Although it is desired that asphalt freezes upon spray application, it is important to prevent cooling of the asphalt material before spraying, while the asphalt is in the distributor. If asphalt freezes in the distributor, the asphalt can cause serious operating problems such as plugging the system and decreasing uniformity of the spray application.
Asphalt distributors conventionally include a tank, a pump, a spray bar and the plumbing network for communicating asphalt from the tank through the pump to the spray bar. The tank, pump and spray bar are conventionally supported directly by a vehicle such as a truck or supported by a detachable trailer pulled behind the vehicle. The plumbing network of an asphalt distributor preferably performs a number of desired functions, including loading of asphalt into the tank, off-loading asphalt out of the tank, transfer to handspray operations, circulating asphalt in the tank during initial asphalt heating, spraying of asphalt, and circulating asphalt through the spray bar while not spraying to prevent freezing of asphalt therein.
A prior attempt of providing an asphalt distributor is exemplified by Hill, U.S. Pat. No. 4,274,586. Hill provides a circulating system that includes dual feed lines connecting the pump to the spray bar, each feed line being connected near one end of the spray bar. In Hill, flow through one feed line is positive or one directional while the flow through the other feed line is positive or negative (bi-directional) depending upon whether spraying or circulation through the bar is desired. During normal spraying operations, flow through both feed lines is positive to deliver asphalt flow to the spray bar. However, when the distributor is stopped, flow through the bi-directional feed line is typically reversed to circulate asphalt through the spray bar and back to the tank to continuously move the asphalt through the feed lines and spray bar and prevent asphalt from freezing therein. Switching the flow is accomplished with an intermediate conduit having an on/off valve therein selectively connecting the feed lines, an adjustable pressure relief valve in the bi-directional feed line, and a pair of on/off valves in the spray bar.
Problems existing in the art relate to the complexity and cost of providing the circulating network in the asphalt distributor. Prior attempts have typically required complex and multiple valves and extensive lengths of circulating plumbing to reverse the flow of asphalt in one of the feed lines and provide the desired operating functions of an asphalt distributor, while all the time preventing asphalt from freezing and plugging the system. Not only are complex valves expensive but the multiple valve locations which are dictated by the routing of interconnecting plumbing do not provide easy operation or straightforward understanding of operation. For manually operated valves, this requires extra worker training and presents a potential safety hazard. The multiple connections can be prone to assembly difficulties and leaks, and the multiple lengths of exposed plumbing result in excessive heat loss from the asphalt which can lead to freezing or plugging of the system.
It is therefore the general aim of the present invention to provide an improved approach of directing asphalt flow through the circulating system of an asphalt distributor.
It is another general aim to centralize the control of asphalt flow through the circulation system of an asphalt distributor.
It is another general aim of the present invention to reduce the cost of providing control valving for the circulating system of an asphalt distributor.
It is therefore objects of the present invention to reduce the complexity of valving in a circulating system of an asphalt distributor while providing for multiple desire operating features.
It is a further objective of the present invention to provide a circulating system in an asphalt distributor that is easier to use, and therefore which is safer to workers.
It is a subsidiary object of the present invention to reduce the lengths of exposed plumbing in a circulating system of an asphalt distributor.
It is another subsidiary object of the present invention according to a preferred embodiment to eliminate the need to reverse the flow in one of the feed lines to the spray bar in the circulating system of an asphalt distributor.
It is therefore a feature of the present invention to provide a directional control valve that has three positions corresponding to four different operating modes of an asphalt distributor. The directional control valve includes an inlet receiving asphalt from a pump, a first outlet to the tank and a second outlet to the spray bar. Control means is provided for selectively positioning the directional control valve. The directional valve includes the first position connecting the inlet to the first outlet for recirculation mode, a second position connecting the inlet to the second outlet for spraying and spray bar circulation modes, and a third position in which the inlet is disconnected from both outlets.
It is an aspect of the present invention that the directional control valve provides a third outlet to a transfer line for operating as off-loading and/or handspray operations. This allows asphalt to continually flow through the directional control valve during transfer operations so as to further prevent freezing of asphalt in the directional control valve.
It is another aspect of the present invention that the directional control valve does not switch the flow through the feed line assembly which delivers asphalt to and from the spray bar. According to the preferred embodiment, the feed line assembly includes a delivery line for one directional delivery flow to the spray bar and a return line for one directional return flow back to the tank. The return line runs coaxially or otherwise adjacent to the delivery line to form a heat exchanger so that static asphalt in the return line is heated by the delivery line to prevent asphalt freezing therein.
It is another feature of the present invention to provide a modular control valve assembly which includes a directional control valve and a pressure relief valve housed in the same valve body. The valve body includes an inlet to the pump, a recirculating outlet for recirculating asphalt to the tank, a delivery outlet for delivering asphalt to the spray bar and a return inlet connected to the spray bar for receiving circulated asphalt from the spray bar. The directional control valve has three positions for alternatively connecting the inlet to the recirculating and delivery outlets. The pressure relief valve is interposed between the return inlet and the recirculating outlet. The pressure relief valve has open and closed positions corresponding to spraying and spray bar circulating modes. During the spraying mode the pressure in the spray bar is low because nozzles on the spray bar are open for discharging asphalt. During spray bar circulation mode, the pressure in the spray bar increases as the nozzles are closed. This causes the pressure relief valve to open thereby allowing asphalt to circulate through the manifold return line and recirculating outlet back to the tank.
It is an aspect of the present invention that the delivery line and return line of the feed line assembly which connects the modular control valve assembly to the spray bar are coaxial or otherwise run adjacent to provide a heat exchanger means for heat transfer therebetween. During the spraying mode, heat is transferred from the delivery line to the return line to prevent freezing of residual or remaining asphalt therein.
These and other objects and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.